This invention relates to a friction device suitable for an aircraft braking system and more particularly to a new and improved pressure plate for use with a multi-disk brake stack, especially one made of ferrous or other metal, in a multiple actuator system for an aircraft wheel and brake assembly. During braking of an aircraft, the axially aligned, alternately splined stator disks and rotor disks of the brake stack are brought into axial sliding engagement with each other, generating considerable heat energy that is dissipated over time. The brake stack is a heat sink which absorbs heat generated during braking action. However, in an aborted or rejected take-off there is a much greater problem of generated heat due to the facts that (a) speed may be higher than normal speed at which the brakes are applied after landing, (b) weight is higher due to fuel provided for consumption during normal flight and (c) only limited or inadequate stopping distance may be available. One proposed solution has been to provide for extended actuating piston travel in such an emergency to compensate for the considerable brake lining wear and degradation and corresponding brake failures, hoping to provide maximum stopping effort. Another alternative is to increase the size of the brake stack and its associated mass, which would also increase the weight of the braking system by a significant amount which is undesirable in meeting overall performance requirements.
The present invention approaches this problem by the provision and use of an annular rigid disk in front of the friction elements of the brake stack which in certain preferred embodiments also includes a steel pressure plate, thus locating this rigid disk between the pressure plate and the plurality of circumferentially spaced actuating pistons which are mounted on the stationary brake carrier that also supports the torque tube.
In any braking action in which the steel pressure plate temperature exceeds 800 degrees Fahrenheit, the steel pressure plate may begin to yield and bend or otherwise deform at the applied clamp force. In an aborted or rejected take-off condition, the brake stack and its adjacent components may reach an elevated temperature of approximately 2000xc2x0 Fahrenheit, which causes the friction materials, which are commonly made from copper based or iron based materials, to melt and the steel portions to loose a significant amount of their strength, resulting in a significant loss in the axial dimension and mass of the brake stack as well as distortion in the components of the brake stack, particularly at the pressure application points of the actuator pistons. This type of action also flexes the rotating disks of the brake stack causing structural deterioration as well as reducing the clamp load efficiency by loss of uniform load application which reduces the torque output capability of the brake. The present invention overcomes these deficiencies by reducing the piston stroke consumption and increasing the clamp load efficiency by utilizing the previously mentioned rigid disk, which rigid disk provides stiffness to the brake stack at elevated temperatures and thus also maintains a more uniform clamp load distribution across the full face of the brake stack and enhances the structural integrity of the brake stack. By utilizing the structural combination of the invention, there is a reduction of piston stroke consumption during high energy braking, thereby reducing the reserve stroke required to accomodate rejected take-off. For the purpose of this specification including the appended claims, high energy is defined as a braking event in which the temperature of pressure plate exceeds 800 degrees Fahrenheit. The assurance of actuator input pressure being better distributed throughout the brake stack by use of the new pressure plate design permits reduction of clamp force and thus piston area for a given frictional material which in turn reduces brake mass and weight, or in the alternative it can allow for the use of a lower coefficient of friction material that otherwise could not be used. The resulting reduced piston to pressure plate contact area reduces undesired conductive heat transfer into the hydraulic system.
According to an aspect of the invention there is provided a friction device comprising: a brake stack having a front axial end adapted to be positioned adjacent to an actuator and a rear axial end adapted to be positioned adjacent to a reaction member, the brake stack including alternating rotor and stator disks mountable with respect to an inner torque tube and an outer wheel for relative rotatable and axial movement, each rotor disk adapted to be coupled to said wheel for rotation therewith and each stator disk adapted to be coupled to said torque tube against rotation relative to said torque tube; the majority of the disks of the brake stack being formed of a material that will deform or flow during an anticipated high energy braking action; the front axial end of the brake stack comprising a first rigid disk; the first rigid disk capable of evenly distributing the clamping load across the faces of said brake stack when said actuator is operated to effect said braking action. The friction device may further include a second rigid disk or rigid reaction disk at the rear axial end of the brake stack. The friction device may further include an actuator adapted to be operatively connected against rotation during a braking action to a fixed mounting means including an axle; a wheel member adapted to be operatively connected to said axle and rotatable with respect thereto; said wheel member having a plurality of axially extending splines; a torque tube member operatively connected to said actuator; said torque tube member having a radially outwardly extending annular end portion defining a torque plate; said torque tube having a plurality of axially extending splines; each rotor disk being coupled to said wheel for rotation therewith and each stator disk being coupled through said torque tube to said wheel support against rotation; the majority of said disks of said brake stack being formed of ferrous material; wherein an annular disk of friction braking material is secured to one face on said pressure plate that is furthermost from said front axial end; and said first rigid disk is formed of a material capable of maintaining a clamp load during braking application during a high energy stop that is more uniform than the clamp load that results when using only a pressure plate of steel. One or both of the first and second rigid disks may be formed of carbon or ceramic material. A particularly preferred material for the first and second rigid disks is carbon fiber reinforced carbon matrix composite.